Process for the preparation of high octane gasoline fractions

ABSTRACT

High-octane gasoline fractions are prepared by an integrated catalytic-cracking process/thermal-cracking process operation. More particularly, high-octane aromatic petroleum fractions are prepared by treatment of the effluents from a catalytic-cracking process and a thermal-cracking process.

United States Patent 1 [111 3,763,034

Kett et al. Oct. 2, 1973 PROCESS FOR THE PREPARATION OF 2,761,825 9/1956 Schultz 208/50 HIGH OCTANE GASOLINE FRACTIONS 2,905,8l7 9/1959 Jahnig et al... 208/55 3,537,975 ll/l970 Blaser 208/50 [75] Inventors: Terrence K. Kett, Boonton; 3,630,886 12/1971 Deed et al. 208/96 Richard A. Reitz, Mendham, both N. f J Primary Examiner-Delbert E. Gantz [73! Asslgneez Esso Research and Engineering Assistant Examiner-G. E. Schmitkons Company, Linden, NJ. Attorney-Leon Chasan et al.

[22] Filed: Feb. 3, 1972 [21] Appl. No.: 223,094 [57] ABSTRACT High-octane gasoline fractions are prepared by an inte- 52 US. l I 1 C 7 5 5 grated catalytic-cracking process/thermal-crackmg [5 1] Int Cl C10 37/04 6 37/06 process operation. More particularly, high-octane aro- 58] Fie'ld g 208/7 89 matic petroleum fractions are prepared by treatment of 2 the effluents from a catalytic-cracking process and a thermal-cracking process.

[56] References Cited UNITED STATES PATENTS 18 Claims, 2 Drawing Figures 2,727,853 12/1955 Hennig 208/53 EVER/770R M0 22 1 K11 K16 LIGHT 500s nun/mo w S' flR/I T/ON UN! 7 430-450; g 547 Fl] (24 3.7 33 flmncr/vz J! 4 DIST/LL17)?! .9 IIlMflT/r fllL/IE W 15' FMur/mv 650-700? I 37 (Z3 I ans row F ,v 2 Rfl 70 fil'fll egg/3722f c6- sorrons A 4 I MIKE UP 765x271; CS/M? Hum? BUM/El? cnr/uvsr 1 F55 z j flywvaeamnow 00F /45 SECOND FREE 7' 10497717 .950; 4 as: ma 5 PATENTEB 2 Sii'EET PROCESS FOR THE PREPARATION OF HIGH OCTANE GASOLINE FRACTIONS The invention relates to the production of high octane lead-free gasoline by cat cracking, thermal cracking and hydrogenation, and more particularly to the integration of cat cracking and thermal cracking processes with aromatic extraction and hydrogenation steps in order to obtain high octane feedstocks.

Since it may be necessary to reduce or eliminate the use of lead compositions in fuels, refiners are considering means of providing high octane fuel components to maintain octane levels adequate for existing high compression automotive engines. If lead is eliminated, the octane loss would be about 3-10 numbers, and additional processing of refinery gasoline components will be desired to maintain the octane number of the fuels at the required levels. The refinery processing units that are currently available to raise octanes are alkylation, reforming, isomerization, catalytic cracking and hydrocracking. Alkylation provides high octane paraffinic hydrocarbon components and reforming provides high octane aromatic components.

Since cat cracking is the major processing tool employed in modern refineries to reduce molecular weight, it would be desirable to use cat cracking as the basic step in providing the maximum quantity of materials that can be used directly in gasoline and/or upgraded by further inexpensive treating steps to provide high octane gasoline components.

A processing technique has been developed in which segregated catalytic cracking steps and extractive distillation are integrated to form high octane naphtha and large yields of alkylation feedstocks. Briefly stated, the process involves the steps of catalytically cracking a virgin petroleum feedstock in a transferline type cracking zone, catalytically cracking cycle stock in a riserdense bed cracking zone, fractionating the combined cracked effluents,recovering alkylation feedstock and employing extractive distillation to recover a high octane monocyclic aromatic hydrocarbon fraction which is suitable for use in motor fuel.

Fresh feed boiling in the gas oil range is mildly catalytically cracked under conditions which maximize the formation of C olefins and which minimize the formation of gas and coke. A segregated transferline cracking unit is particularly suited to this type of cracking operation because it provides high selectivities to olefins. The highly active crystalline molecular sieve catalysts give maximum conversion of the feed in the transferline and they are preferred for this cracking step. The catalytically cracked effluent is separated into a C containing fraction that can be further processed to obtain feed for an isoparaffin alkylation unit, and a gasoline boiling fraction, which has a nominal boiling range of from about 100F to about 430F (430F), containing gasoline boiling range paraffms, olefins, naphthenes and monocyclic aromatics. The monocyclic aromatic hydrocarbons boiling in the gasoline boiling range may have side chains containing up to six carbon atoms attached to the benzene ring. These are desirable high octane components for gasoline. These aromatics are separated by extractive distillation; and, in a preferred embodiment, the extractive distillation solvent is produced in situ by recovering it from the cat fractionator. The fraction boiling in the range of from about 650F to 700F comprises three ring aromatic hydrocarbons and this material will selectively separate the desired monocyclic aromatic hydrocarbons from the nonaromatic material. The monocyclic aromatics can then be recovered from the high boiling solvent fraction by a simple flashing step. The non-aromatic material can be recovered or part of it can be recovered, but preferably this fraction is recycled for further cracking. If desired, however, the 300F to 430F fraction of the nonaromatic raffinate can be used directly for a high quality jet fuel blending material. If the monocyclic aromatics are not separated from the material that is exposed to further cracking as recycle, the result is further condensation in the cracking unit to aromatics which boil above the gasoline boiling range and cracking of monocyclic aromatic side chains to gas. Furthermore, as the aromatic rings condense, hydrogen atoms will be transferred to olefins to form saturates, reducing the quantity of desirable low molecular weight olefin product.

In order to obtain incremental conversion, a recycle stream is catalytically cracked. The cycle oil is more difficult to crack than the fresh feed and therefore, a dense bed is used to provide increased severity. Thus, a cycle oil containing predominantly multi-ring aromatic hydrocarbons having one or more side chains of 1-10 carbon atoms is catalytically cracked to selectively remove the side chains, producing large quantities of C -C olefins.

It has now been found that by the integration of segregated catalytic cracking and thermal cracking with hydrogenation and aromatics'extraction, high octane aromatics and alkylation feedstocks can be produced from virgin petroleum feedstock and a coker feed such as crude vacuum or atmospheric distillation bottoms, crude oil, heavy residua from a fractionator, pitch or asphalt.

In accordance with the present invention a high octane aromatic petroleum fraction is prepared by means of a process in which a fresh petroleum feedstock is catalytically cracked in a transferline cracking reactor to provide a first cracked effluent, a recycle petroleum fraction is catalytically cracked in a riser-dense bed cracking reactor to provide a second cracked effluent and the two effluents are combined. The combined effluents are passed to a first fractionator and at least two recycle fractions, a fraction nominally boiling in the range between about 650F and 700F, and a fraction boiling below about 400F450F are recovered. The overhead fraction is separated into a hydrocarbon fraction containing substantial quantities of C -C hydrocarbons and a hydrocarbon fraction containing substantial quantities of monocyclic aromatic hydrocarbons which are extracted by extractive distillation.

A residuum feedstock is thermally cracked in any suitable thermal cracking reactor, such as a visbreaker or a fluid or delayed coking reactor, and the effluent from the reactor is passed to a second. fractionator. At least an overhead fraction boiling below about 400-450F, a recycle fraction boiling above about 900-950F, and a fraction boiling in the range from above about 400-450F to below about 900-950F are recovered. A portion of the latter fraction may be hydrogenated and combined with the fresh petroleum feedstock to the catalytic cracking transfer line reactor. The nonhydrogenated portion is passed to the catalytic cracking dense bed reactor. The overhead fraction from the second fractionator is separated into a hydrocarbon fraction containing substantial quantities of C, to C hydrocarbons and a hydrocarbon fraction containing substantial quantities of monocyclic aromatic hydrocarbons. The latter fraction is subjected to the above noted extractive distillation.

Although the invention is described below with specific regard to thermal cracking by fluid coking and delayed coking, it is to be understood that the invention is not limited thereto, but can employ any thermal cracking process.

The advantages and objects of the invention will become apparent from the following detailed description, particularly when read together with the drawings wherein:

FIG. 1 is a diagrammatic flow sheet illustrating an embodiment of the invention wherein the thermal cracking is accomplished by a fluid coking reactor; and,

FIG. 2 is a diagrammatic flow sheet illustrating another embodiment of the invention wherein the thermal cracking is accomplished by a delayed coking reactor.

Referring to FIG. 1, fresh feed is fed by line 1 to the lowermost part of transferline reactor 2. The feed is mixed with regenerated catalyst flowing in return line 3. Fresh makeup catalyst is added via line 4. Suitable fresh cracking feedstocks comprise hydrocarbon fractions boiling in the range of 450F to l 100F, preferably 550F to 950F. Preferred fresh petroleum feedstocks include virgin atmospheric gas oils, virgin vacuum gas oils, hydrotreated gas oils, coker gas oils, fractions from solvent extraction, deasphalted oils and mixtures thereof. The preferred catalysts for the present process are the crystalline aluminosilicate zeolite types. In general, the chemical formula of the anhydrous crystalline zeolites employed in the present invention, expressed in terms of moles, may be represented as:

0.9 i 0.2 Me O A1 XSiO wherein Me is selected from the group consisting of metal cations, hydrogen and ammonia, n is its valence and X is a number in the range of 2 to 14, preferably 2.5 to 6.5. The crystalline aluminosilicate zeolites include synthetic crystalline aluminosilicates, naturally occurring crystalline aluminosilicates, and caustic treated aged clays in which a portion of the clay has been converted to crystalline zeolite. Synthetic materials include faujasites and mordenites. Natural materials are erionite, analcite, faujasite, phillipsite, clinoptilolite, chabazite, gmelinite, mordenite and mixtures thereof. Montmorillonite and kaolin clays can be treated to obtain crystalline aluminosilicates. All or a portion of the cations of the zeolites such as sodium cations can be replaced with hydrogen ions, ammonium ions, or metal cations such as rare earths, manganese, cobalt, zinc and other metals of Group I to VIII of the Periodic Table. Matrix type fluid cracking catalysts in which the zeolite crystals are coated with or encapsulated in a siliceous gel are preferred zeolite type catalysts.

The mixture or dilute suspension of fluidized catalyst and feed in vapor or mixed vapor-liquid phase passes upwardly through transferline reactor 2 at a velocity in the range of from about 6 to about 50 ft. per second. The length-to-diameter ratio (L/D) of the reactor ranges from about 4 to about 50. The space velocity is in the range of to 1800 w/hr/w and preferably 50-l 50 w/hr/w. Because the mixture of regenerated zeolite catalyst and fresh zeolite catalyst is very active, the fresh feed in transferline 2 is cracked in a few seconds, i.e., less than about 30 seconds, and more probably 0.5-l0 seconds.

Effluent from the transferline reactor is initially separated in rough cut cyclone 5. Separated catalyst passes down the dipleg into the dense bed and cracked effluent passes up through the disengaging zone to cyclone 6.

Segregated cycle oil, the source of which will be discussed hereinafter, is fed by line 7 into the bottom of riser 8. The cycle oil is mixed with regenerated catalyst from regenerator 9. The cycle oil is cracked in part in riser 8 and in part in fluidized dense bed 10. Cracked effluent passes through cyclone 6 into line 11 and catalyst is returned to the dense bed via the dipleg. Spent catalyst from the transferline cracking step and from the dense bed is stripped in stripper 12 and passed by line 13 to regenerator 9. Regeneration is conventional and generally the amount of carbon on the regenerated catalyst will range from about 0.02 to L0 wt. percent, based on the weight of the catalyst. Flue gas exits via line 14.

Cracking conditions in the transferline cracking zon and in the dense bed include temperatures in the range of 850F. to 1050F and pressures in the range of 5 to 35 psig. The cycle oil is subjected to more severe cracking action because of the effect of the relatively lower space velocity; i.e., less than about 30 w/hr/w.

The cracked effluent from the two catalytic cracking zones is passed by line 11 to a first fractionator 15. An overhead fraction having an end point in the range of about 390F to 430F is taken overhead from the first fractionator by line 16. This fraction contains substantial quantities of C C hydrocarbons used in isoparaffin alkylation and substantial quantities of monocyclic aromatic hydrocarbons which boil in the gasoline boiling range. The overhead fraction is passed to a light ends separation unit 19. The light ends unit is operated in the conventional manner to provide any desired type of separation. In this particular embodiment, for example, a gas fraction including C minus hydrocarbons and other gases is recovered by line 20. Propane and n-butane are recovered by line 21. A fraction containing C and/or C olefms; i.e., propylene and pentenes, can be fed by line 22 to the alkylation unit, or alternatively any part of this fraction can be recovered by line 23. A C fraction containing butenes and isobutane is passed via line 24 to isoparaffin alkylation unit 25. Alkylation is a conventional operation with catalysts such as H and HP at temperatures in the range of 20F to F and pressures in the range of 2 to psig. A hydrocarbon fraction containing substantial quantities of monocyclic aromatic hydrocarbons and typically boiling in the range of from about 1 15F to 410F is passed from the light ends separation unit 19 by line 26 to the lower section of extractive distillation tower 27. This predominantly monocyclic aromatic hydrocarbon fraction is contacted with a solvent under extractive distillation conditions. In extractive distillation the separation of different components of mixtures which have similar vapor pressures is effected by flowing a relatively high boiling solvent, which is selective for one of the components in the feed, down a distillation column as the distillation proceeds. The relatively less soluble component passes overhead, while the selective solvent scrubs the soluble component from the feed.

The solvent containing the dissolved component is withdrawn from the bottom of the column and the dissolved component and solvent may be separated in an auxiliary unit. Tower 27 can be operated at temperatures in the range of 250F to 500F and pressures in the range of O to psig. Conventional features of extractive distillation such as reboiler elements, reflux systems, bleed streams and pumparounds have not been shown. 3

Any suitable solvent for monocyclic aromatic hydrocarbons can be used; however, it is a feature ofthe invention that the solvent can be obtained from the process itself rather than from an external source. Specifically, the solvent can be obtained by recovering a particular fraction from the first fractionator 15. In a preferred embodiment, the solvent is an aromatic hydrocarbon fraction containing a major amount of three ring aromatic hydrocarbons. Thus, line 28 passes an aromatic fraction boiling in the range of from about 650F to about 700F to the upper portion of tower 27. The multi-ring aromatic fraction passes downwardly through the tower extracting monocyclic aromatics from the extractive distillation feedstock. The extract fraction is passed via line 29 to flash tower 30. The monocyclic aromatic gasoline fraction is flashed overhead from tower 30 for recovery by line 31. The monocyclic aromatic hydrocarbon fraction recovered from the process will have an unleaded research octane number of from about 96 to about 102, generally about 100. The solvent is recycled via line 32. The nonaromatic raftinate from tower 27 can be recycled to the dense bed cracking step by line 33 and/or any portion of it can be recovered by line 34 as a product of the process.

Since two ring aromatic hydrocarbons (430-480F) with fewer than three carbon side chains are not desirable recycle materials, an aromatic fraction of this type is removed from the process and passed to the hydrotreater by line 35.

In this embodiment the cycle oil comprises two components from fractionator 15. A fraction boiling in the range of about 700F to 800F is passed by line 36 to line 7 for admixture with a fraction boiling in the range of about 480F to 650F and the mixed fractions are recycled to the dense bed reactor. The composition of the cycle oil is optional and any fraction or mixture of fractions amenable to dense bed cracking can be recycled for this purpose.

, A residua feed 40 is introduced into a fluid coking reactor 42, which holds a fluid bed of coke particles. The fluid coking feeds can be crude vacuum of atmospheric distillationbottoms, or crude oil heavy residua from a fractionator.

The fluid bed material 44 can also be inert particles such as silica, alumina, zirconia, magnesia, alundum, mullite, synthetically prepared or naturally occurring The reactor temperature is controlled by the flow of hot coke 46, from the burner 48. Steam 50 can be introduced at the bottom of the reactor 42 to strip the coke to the burner and fluidize the bed 44.

In the fluid coker reactor 42, mixing is good and the feed distributes uniformly over the surface of the particles 44. The feed cracks and vaporizes leaving a residue of coke. The volume of vapors increases progressively up through the bed due to the formation of cracked products. To take advantage of this, the reactor is built in the form of an inverted cone, so that less aeration steam is needed to maintain a given minimum velocity in the bed 44. Vapor products leave the bed 44 and pass through cyclones which remove most of the entrained coke. The vapors then discharge into the bottom of the scrubber section 52 of the reactor 42. The remaining coke dust is then scrubbed out and the products are cooled to condense out the heavy tar. The resulting slurry is recycled to the lower part of the coking reactor so that it is subjected to maximum cracking intensity. An overhead fraction is taken from the scrubber 52 by line 54. V

In the reactor 42, the coke particles flow down through the vessel into a stripping zone at the bottom. Stripping steam vaporizes and displaces product hydrocarbons between the coke particles. The coke then flows down a standpipe and through a slide valve which controls the reactor bed level. A riser carries the coke up to the burner 48. Steam is added to the riser to reduce the density and induce upward flow.

The average bed temperature in the burner of l100 to 1350F is maintained by adding air as needed to produce a required rate of burning of part of the product coke. Flue gases from the bed pass through cyclones 56 and discharge to the stack through a variable orifice which controls burner pressure. Hot coke from the burner bed 58 is returned to the reactor 42 through a standpipe, slide valve and riser 46.

Since coke is one of the products of the process it must be withdrawn from the system in order to keep the solids inventory from increasing. The coke product is removed from the burner bed through a quenchelutriator drum. Water is added to the latter to cool the coke, and make steam which entrains the fine particles and carries them back into the burner. Cooled coarse coke is withdrawn and sent to storage or sent to a steam gasifier to produce a hydrogen rich gaseous stream.

Thermal cracking of the pitch leaves a coke residue, which deposits as a layer on the particles in the reactor. The particles grow in size as new coke is laid down. In order to keep the circulating coke from getting too coarse, large particles are removed through the elutriator and replaced with small seed particles. The number of seeds required is theoretically equal to the number of coarse coke particles withdrawn as product. A simple grinding system in the reactor supplies the seeds.

material such as pumice, clay, kieselguhr, diatomaceous earth, bauxite, but coke particles having diameters in the range from 40 to 400 microns are preferred. The fluid coking operation can be carried out at a temperature in the range from 900 to 1150F, but in this embodiment is preferably in the range from 1000 to ll50F. Superatmospheric pressures can be used, preferably from about 5-35 psig.; and the gas flow is maintained at a superficial velocity of 0.2 to 5.0 feet/- second in order to have good fluidity of the bed.

This consists of a number of supersonic steam jet attriters.

The primary products from the coking operation are separated in the coker fractionator 58. A bottoms fraction boiling at a temperature of at least 800F and preferably 950F+ is recycled to the reactor 42 via line 59. The heavy gas oil fraction is removed from the fractionator 58 by line 61 and is fed to the catalytic cracking dense bed reactor 10 via riser 8. The heavy gas oil fraction boils in the range from 550F plus and preferably from 650950F.

' I ture fluid coking, for example, at temperatures of about llF, is especially rich in one-ring aromatics suited for high octane gasoline and chemicals.

AROMATIC AND OLEFIN CONTENT OF C /650F COKER PRODUCT Feed Weyburn-Midale 925F+ Bottoms Fluid Reactor (Temp., F) 1120 1107 1035 968 C /3 F Benzene (LV% of cut) 16.2 12.3 5.4 1.7 Product Toluene 30.4 19.4 10.5 3.8 C; Aromatics 4.6 11.9 8.5 4.3 C, Aromatics 2.6 1.1 1.0 0.9 Mono-olefins 16.9 36.2 52.5 66.3 Diolefins 10.2 14.3 14.8 10.6 Motor Octane No. 82 78 (3cc Tel) 3l0 Benzene 0.6 0.4 0.2 0.1 430F (LV% of cut) Product Toluene 2.5 1.0 0.6 0.4 Cg Aromatics 9.6 5.2 1.8 1.1 C Aromatics 20.4 14.3 7.1 4.0 C Aromatics 13.5 12.4 5.7 4.5 C Aromatics 24.9 21.6 9.8 8.3 Olefms 25.9 40.6 66.8 67.3

430 One Ring 28.4 25.6 23.3 21.1 650F Aromatics Product (LV% of cut) Two Ring 59.8 53.2 33.0 21.3 Aromatics Olefins 10.4 21.2 35.9 45.5

The 430F fraction is sent to a condenser 60 via line 62 and the hydrocarbons having less than six carbons are separated from the C hydrocarbons. 1n the preferred embodiment, the latter is sent to the extractive distillation tower 27 by line 64 in order to get the desired high aromatic naphtha; however, to insure against the presence of diolefins in the extract, it can first be sent to a diolefins conversion process such as a hydrotreater or to the catalytic cracker. The stream of C hydrocarbons from condenser 60 can be further processed to recover C -C olefins.

The 650-950F fraction can be hydrogenated in order to improve its quality before being sent to the catalytic cracker.

FIG. 2 shows an alternative embodiment of the present invention wherein thermal cracking is accomplished by delayed coking. As shown in FIG. 2, feed 70 is introduced into the coker product, or second fractionator 72 where it is heated and the light virgin fractions are flashed off.

The fractionator bottoms including a recycle stream of heavy product are heated in a furnace 74 to cracking temperature. The heated oil is carried by line 76 to a soaking drum 78 which provides the long residence time needed for thermal cracking. Cracked products leave at the top by line 80 and coke deposits in the drum 78. To give semi-continuous operation, at least two drums 78 and 78A are used. Thus, while one drum is being cleaned, the other is on-stream.

Recycling of heavy material from the bottom of the fractionator 72 to the reactor 78 serves to give further conversion of heavy fractions in the product and serves to carry heat into the reactor.

A reactor can be expected to operate for about one day before it becomes filled with coke. It is then cleaned hydraulically, using high pressure water jets. The clean reactor 78A is ready to go back on-stream by the time the reactor 78 is full of coke. Wet coke chunks from cleaning are conveyed to a storage pile and drained.

The processing scheme as described supra is noted to have numerous advantages including excellent flexibility. A wide range of cracking severities can be achieved utilizing segregated transferline cracking for fresh feed and segregated dense bed cracking for cycle oil. Variations in the feed compositions, catalyst type, the catalyst to oil ratio, and the cracking temperatures can be made to obtain optimum high octane gasoline components at all times.

One of the major benefits of the process of the invention is that it provides an efficient, inexpensive means of recovering high octane aromatic motor fuel components as well as large quantities of alkylation feed. Recracking of desirable aromatic naphtha components is substantially reduced. Segregated cracking of selected fractions at the conditions that are most suitable for each fraction provides more efficient conversion to materials that can be used to raise gasoline octane without depending on lead compositions for octane boost.

A further benefit is that a quantity of one-ring aromatic compounds with good boiling point distribution and high octane is recovered from the naphtha produced by the thermal cracker for use in gasoline or chemicals manufacture.

The hydrogenation of the two-ring aromatics in the 430-480F catalytic product and the 430F/1BP coker product produces one-ring aromatic compounds which will yield one-ring aromatics in the naphtha boiling range after catalytic cracking.

The low aromatic naphtha, which is recovered from the overhead fraction of the extractive distillation tower can either be recracked in the riser-dense bed reactor to make alkylate feedstock and aromatics or can be sent to a naphtha reforming process after hydrogenation, if required, or can be used for jet fuel or other processes or products.

The nominal 650-700F product from the extraction unit flash tower, which is rich in three-ring aromatics, can be used for product, or for heat balancing of the catalytic cracker by being burned either in a furnace or in the regenerator.

What is claimed is:

1. A process for the preparation of a high octane aromatic petroleum fraction comprising the steps of:

a. catalytically cracking a feedstock comprising fresh petroleum and a hydrogenated fraction boiling in the range of from above about 400-450F to below about 900-950F to provide a first cracked effluent;

b. catalytically cracking a recycle petroleum fraction to provide a second cracked effluent;

c. combining said first cracked effluent and said second cracked effluent to provide a combined effluent;

d. passing said combined effluent to a first fraction ator;

e. recovering at least two petroleum fractions from said first fractionator including:

i. an overhead fraction boiling below about 400-450F; and ii. a recycle fraction;

f. separating said overhead fraction into a hydrocarbon fraction containing substantial quantities of C --C hydrocarbons and a hydrocarbon fraction containing substantial quantities of monocyclic aromatic hydrocarbons;

g. extracting monocyclic aromatic hydrocarbons from the hydrocarbon fraction containing substantial quantities of monocyclic aromatic hydrocarbons by extractive distillation;

h. thermally cracking a residuum feedstock in a thermal cracking reactor to provide a third cracked effluent; v

. passing said third cracked effluent to a second fractionator;

j. recovering at least three petroleum fractions from said second fractionator including:

i. an overhead fraction boiling below about ii. a recycle fraction boiling above about iii. a fraction boiling in the range from, above about 400-450F to below about 900950F;

k. combining at least a portion of said fraction (iii) with said fresh petroleum to form the feedstock for the catalytic cracking of step (b);

. separating said overhead fraction (j) (i) into a hydrocarbon fraction containing substantial quantities of C -C hydrocarbons and a hydrocarbon fraction containing substantial quantities of monocyclic aromatic hydrocarbons;

m. combining the hydrocarbon fraction containing substantial quantities of monocyclic aromatic hydrocarbons of step (l) with the hydrocarbon fraction containing substantial quantities of monocyclic aromatic hydrocarbons of step (f) prior to the extractive distillation of step (g).

2. Process according to claim 1, in which the catalytic cracking of step (a) is carried out in a transferline cracking reactor in the presence of a crystalline zeolite cracking catalyst.

3. Process according to claim 1, in which the catalytic cracking of step (b) is carried out in a riser-dense bed cracking reactor in the presence of a crystalline zeolite cracking catalyst.

4. Process according to claim 1, in which the thermal cracking of step (h) is carried out in a fluid coking reactor, where the reactor temperature ranges from 900-l 150F, preferably from l000-l l50F to maximize the production of one-ring aromatics suited for high octane gasoline and chemicals.

5. Process according to claim 1, in which the thermal cracking of step (b) is carried out in a delayed coking reactor.

6. Process according to claim 1, in which said hydrocarbon fractions containing substantial quantities of C -C hydrocarbons of step (f) and step (1) are subjected to light ends treatment to concentrate said 10 C -C hydrocarbons and said C -C hydrocarbons are fed to an alkylation unit for alkylation in the presence of an alkylation catalyst.

7. Process according to claim 1 in which the catalytic cracking is accomplished by a catalyst comprising a crystalline zeolite encapsulated in a siliceous gel matrix.

8. Process according to claim 1 in which said recycle petroleum fraction of step (e) comprises a major amount of multi-ring aromatic hydrocarbons having side chains of one to 10 carbon atoms.

9. Process according to claim l in which the petroleum feedstock comprises a gas oil.

10. Process according to claim 1 in which step (g) comprises contacting the monocyclic aromatic hydrocarbon fraction with a solvent for aromatic hydrocarbons at extractive distillation conditions, and recovering a monocyclic aromatic hydrocarbon fraction from said solvent.

11. Process according to claim 1 in which the fresh petroleum of the feedstock is a virgin gas oil and the cracking is at mild cracking conditions in a fluidized transferline cracking reactor in the presence of a crystalline zeolite cracking catalyst;

said cracking of said recycle petroleum fraction of step (b) is carried out in a riser-dense fluidized bed cracking reactor in the presence of a crystalline zeolite cracking catalyst; and in step (e) an aromatic fraction containing a major amount of two ring aromatics and a solvent fraction comprising a major amount of three ring aromatic hydrocarbon compounds are recovered.

12. The process of claim 11 in which said overhead fraction of step (f) is separated into an alkylation feed fraction containing a major amount of butenes and isobutane and a predominantly aromatic fraction containing substantial quantities of monocyclic aromatic hydrocarbons.

13. The process of claim 12, in which said recycle fraction of step (j) (ii) is passed to said coking reactor.

14. Process according to claim 13, in which step (j) (iii) further comprises: recovering a fraction boiling in the range from above about 600650F to below about 900-950F; and a fraction boiling in the range from above about 400450F to below about 600-650F.

15. Process according to claim 14, in which step (e) further comprises recovering a third catalytic cracking product fraction boiling in the range from about about 400-450F to below about 450-500F, said third fraction being subjected to a hydrogenation process.

16. Process according to claim 15, wherein the thermal cracking product fraction boiling in the range from above about 400450F to below about 600650F is subjected to the hydrogenation process.

17. Process according to claim 14, wherein the thermal cracking product fraction boiling in the range of from above 600-650F to below about 900-950F is also subjected to the hydrogenation process.

18. Process according to claim 1 wherein all the hydrogenated fractions are combined with said fresh petroleum to form the feedstock for the catalytic cracking of step (a). 

2. Process according to claim 1, in which the catalytic cracking of step (a) is carried out in a transferline cracking reactor in the presence of a crystalline zeolite cracking catalyst.
 3. Process according to claim 1, in which the catalytic cracking of step (b) is carried out in a riser-dense bed cracking reactor in the presence of a crystalline zeolite cracking catalyst.
 4. Process according to claim 1, in which the thermal cracking of step (h) is carried out in a flUid coking reactor, where the reactor temperature ranges from 900*-1150*F, preferably from 1000*-1150*F to maximize the production of one-ring aromatics suited for high octane gasoline and chemicals.
 5. Process according to claim 1, in which the thermal cracking of step (h) is carried out in a delayed coking reactor.
 6. Process according to claim 1, in which said hydrocarbon fractions containing substantial quantities of C3-C5 hydrocarbons of step (f) and step (l) are subjected to light ends treatment to concentrate said C3-C5 hydrocarbons and said C3-C5 hydrocarbons are fed to an alkylation unit for alkylation in the presence of an alkylation catalyst.
 7. Process according to claim 1 in which the catalytic cracking is accomplished by a catalyst comprising a crystalline zeolite encapsulated in a siliceous gel matrix.
 8. Process according to claim 1 in which said recycle petroleum fraction of step (e) comprises a major amount of multi-ring aromatic hydrocarbons having side chains of one to 10 carbon atoms.
 9. Process according to claim 1 in which the petroleum feedstock comprises a gas oil.
 10. Process according to claim 1 in which step (g) comprises contacting the monocyclic aromatic hydrocarbon fraction with a solvent for aromatic hydrocarbons at extractive distillation conditions, and recovering a monocyclic aromatic hydrocarbon fraction from said solvent.
 11. Process according to claim 1 in which the fresh petroleum of the feedstock is a virgin gas oil and the cracking is at mild cracking conditions in a fluidized transferline cracking reactor in the presence of a crystalline zeolite cracking catalyst; said cracking of said recycle petroleum fraction of step (b) is carried out in a riser-dense fluidized bed cracking reactor in the presence of a crystalline zeolite cracking catalyst; and in step (e) an aromatic fraction containing a major amount of two ring aromatics and a solvent fraction comprising a major amount of three ring aromatic hydrocarbon compounds are recovered.
 12. The process of claim 11 in which said overhead fraction of step (f) is separated into an alkylation feed fraction containing a major amount of butenes and isobutane and a predominantly aromatic fraction containing substantial quantities of monocyclic aromatic hydrocarbons.
 13. The process of claim 12, in which said recycle fraction of step (j) (ii) is passed to said coking reactor.
 14. Process according to claim 13, in which step (j) (iii) further comprises: recovering a fraction boiling in the range from above about 600*-650*F to below about 900*-950*F; and a fraction boiling in the range from above about 400*-450*F to below about 600*-650*F.
 15. Process according to claim 14, in which step (e) further comprises recovering a third catalytic cracking product fraction boiling in the range from about about 400*-450*F to below about 450*-500*F, said third fraction being subjected to a hydrogenation process.
 16. Process according to claim 15, wherein the thermal cracking product fraction boiling in the range from above about 400*-450*F to below about 600*-650*F is subjected to the hydrogenation process.
 17. Process according to claim 14, wherein the thermal cracking product fraction boiling in the range of from above 600*-650*F to below about 900*-950*F is also subjected to the hydrogenation process.
 18. Process according to claim 1 wherein all the hydrogenated fractions are combined with said fresh petroleum to form the feedstock for the catalytic cracking of step (a). 